1. Technical Field of the Invention
The invention concerns a film blowing plant for manufacturing films from thermoplastics, having a blow head, above which is disposed an air cooling and behind it in the direction of movement of the film a measuring device for the film thickness, whereby the cooling capacity across the circumference of the tube can be controlled variably by altering the throughput quantity of the air cooling ring and respective adjustment in every region is controlled according to the measurement values of the measuring device.
2. Description of the Prior Art
A device of this generic concept is disclosed in DE-PS 3743, which stems from the same applicant, with which the film thickness is recorded across its circumference via a measuring device and in case of deviations from the target value at the correspondingly assigned place of the circumference of the still flexible blown film, the cooling capacity is altered and adjusted. When the cooling capacity is increased the stretching of the film which takes place in this region of the plant is decreased due to the rapid solidification, so that a comparatively thick wall thickness remains. On the other hand, with a reduction of the cooling capacity the time is extended during which the film can be stretched, resulting in a further reduction of the wall thickness. The cooling capacity is influenced by altering the throughput quantity of the air passing the air cooling ring. In order to execute a varying effect across the circumference of the tubular film, the air cooling ring is subdivided by partition walls extending in a radial direction and thus forming sectors whose upper and/or lower borders' spatial position is changed, so that the cross section available to the cooling air and consequently the throughput quantity in this region is influenced.
In practice, the device of this generic concept has shown itself to be encumbered with considerable disadvantages. On the one hand, the partition walls only permit a sharp alteration in the cooling performance across the circumference of the tubular foil, which, however, should be correspond to the conditions and seen from the point of view of the circumference of the tubular film should be comparatively continuous. Furthermore, the partition walls result, due to flow-dynamic reasons, in the formation of vortexes and other irregular point-by-point flows, which means an undefined and largely uncontrolled influencing of the cooling capacity.